Aluminium Alloys: Wrought and Cast Designations, Temper Codes, and Applications

Aluminium is the second most widely used structural metal on Earth, surpassing all others except steel. Its combination of low density (2.70 g/cm³), excellent corrosion resistance through a self-healing Al₂O₃ passive film, high electrical and thermal conductivity, and the capacity to achieve a very wide range of mechanical properties through alloying and heat treatment makes it indispensable in aerospace, automotive, marine, packaging, and construction engineering. The Aluminum Association (AA) wrought designation system and the three-digit cast designation system provide a standardised, internationally recognised framework for communicating alloy composition and heat treatment condition. Understanding this framework precisely is essential for correct materials selection, procurement, and quality assurance.

The Aluminium Wrought Alloy Designation System

The four-digit designation system for wrought aluminium alloys is administered by the Aluminum Association (AA) in the USA and adopted by ISO, EN, and most national standards bodies worldwide. Under EN, the designation is typically preceded by “EN AW-” (e.g., EN AW-6082), where AW denotes aluminium wrought. Under the older British Standard, the prefix was “BS L” followed by a code number. The AA system governs the relationship between composition limits and alloy number.

The Eight Wrought Alloy Series

Precipitation Hardening: Theory and Thermodynamics

Precipitation hardening (age hardening) is the most powerful strengthening mechanism available for aluminium alloys. It exploits the decreasing solid solubility of alloying elements in aluminium with decreasing temperature, as defined by the solvus boundary on the binary phase diagram.

Thermodynamic Basis and the Solvus

In the Al-Cu binary system, the maximum solid solubility of copper in aluminium is approximately 5.65 wt% Cu at the eutectic temperature (548°C), decreasing to less than 0.5 wt% at room temperature. For a 2024 alloy (4.35% Cu), the solvus temperature is approximately 500°C. Solution heat treatment at 495–510°C dissolves all copper into the aluminium FCC matrix, creating a single-phase solid solution. Rapid quenching to room temperature suppresses the equilibrium precipitation of CuAl₂ (θ phase), retaining all the copper in metastable solid solution — the supersaturated solid solution (SSSS). This supersaturation is the thermodynamic driving force for all subsequent precipitation.

The Precipitation Sequence in Al-Cu

The transformation of SSSS to equilibrium θ-CuAl₂ does not occur in a single step. A series of metastable phases form sequentially, each more stable than the last but less stable than θ, because nucleation of the fully incoherent equilibrium phase faces an enormous interfacial energy barrier. The sequence is:

SSSS  →  GP1 zones  →  GP2 zones (θ′′)  →  θ′  →  θ (CuAl₂)

GP1 zones:
  • Cu-enriched discs on {001}α planes; 1–2 atom planes thick; diameter 3–10 nm
  • Fully coherent with Al matrix; produce coherency strain field (main hardening)
  • Form spontaneously at room temperature (natural ageing)

GP2 zones / θ′′:
  • Thicker ordered Cu-Al layers; 2–5 nm thick; up to 30 nm diameter
  • Fully coherent; stronger lattice distortion; maximum coherency hardening
  • Form during early artificial ageing at 100–150°C

θ′ (Al₂Cu):
  • Semicoherent on {001}α; 10–100 nm diameter, 1–4 nm thick
  • Nucleates heterogeneously at GP2/matrix interfaces and dislocations
  • PEAK hardness / strength (T6 condition); maximum impedance of dislocation motion

θ (CuAl₂):
  • Fully incoherent equilibrium phase; tetragonal structure
  • >100 nm; coarsens rapidly (Ostwald ripening) above 200°C
  • Overaged condition; lower strength; T73/T76 for SCC resistance

The strengthening arises primarily from two mechanisms. In the coherent GP zone and θ′′ stage, coherency strain hardening dominates: the lattice distortion around the precipitate imposes a long-range stress field that resists dislocation passage. In the θ′ stage, precipitate cutting and Orowan looping are competitive: fine θ′ particles are cut by dislocations (requiring antiphase boundary energy), while coarser particles are bypassed by Orowan dislocation loops, leaving residual loops that back-stress subsequent dislocations.

Precipitation in Other Alloy Systems

Each alloy series has its characteristic precipitate sequence, governed by the specific phase diagram of the principal alloying system:

In 6xxx alloys, the primary hardening precipitate is β′′ (Mg₅Si₆, monoclinic, needle-shaped on <100> directions). It provides peak T6 hardness in 6061 (Vickers hardness ~105 HV after 160°C/8 h). Excess silicon beyond the Mg₂Si stoichiometric ratio forms additional Si precipitates and improves strength at the cost of slightly reduced ductility. The 6xxx series is uniquely suited to hot extrusion because β′′ can be precipitated directly during the controlled quench on the extrusion press exit (the “press quench”), eliminating a separate solution heat treatment step for many profiles.

In 7xxx alloys, the major strengthening precipitate in peak T6 condition is η′ (MgZn₂, hexagonal, semi-coherent), formed at temperatures of 100–130°C. The η′ → η transition at higher temperatures or longer times produces overageing and the SCC-resistant T73 temper. Copper additions in alloys like 7075 (1.2–2.0% Cu) partition to grain boundaries during the quench and suppress preferential grain-boundary precipitation of the η phase, significantly improving SCC resistance relative to the binary Al-Zn-Mg system. The ratio of precipitate-free zone (PFZ) width to grain size is a key microstructural parameter controlling SCC susceptibility in 7xxx alloys.

The Temper Designation System

The temper designation, separated from the alloy number by a hyphen, specifies the mechanical and thermal processing history that determines the property condition. It is defined in ANSI H35.1 (USA), ISO 2107, and EN 515. The temper applies after the alloy number: 6061-T6, 5083-H321, 1100-O.

Stress-Relief Suffixes

Stress-relief suffixes follow the T number and indicate additional processing to reduce residual stresses from quenching. These are important for machined structural parts because unrelieved quench stresses can cause distortion during machining. The most common:

• Tx51: Stress-relieved by controlled stretching (1–3% permanent set for plate; 1–3% for rolled or cold-finished bar/rod/wire). Most common for plate: 7075-T651, 2024-T351, 6061-T651.
• Tx52: Stress-relieved by controlled compression (1–5% permanent set). Used for forgings and thick plate where stretching is impractical.
• Tx54: Stress-relieved by both stretching and compressive straightening. Used for die forgings.
• Tx510, Tx511: Extrusions stress-relieved by stretching without (510) or with (511) subsequent straightening.

The Alloy Series in Technical Detail

1xxx Series — Commercially Pure Aluminium

Alloys in this series contain at least 99.00% aluminium (1100), with higher-purity grades for specific applications: 1050 (99.50%), 1070 (99.70%), 1199 (99.99%). The principal alloying element is either iron or silicon (as impurity elements): both form insoluble intermetallic compounds (Al₃Fe, Al_xFe_ySi) that pin grain boundaries and provide modest strengthening. Alloy 1350 (99.50% Al minimum, tightly controlled Fe, Si, Cu for maximum conductivity) is the standard alloy for electrical overhead conductors (ACSR — aluminium conductor steel reinforced, and AAAC — all-aluminium alloy conductor). Its electrical conductivity is approximately 61% IACS (International Annealed Copper Standard). Strengthening of 1xxx alloys is limited to work hardening (H tempers); typical UTS ranges from 70 MPa (O temper) to 165 MPa (H18).

2xxx Series — Aluminium-Copper

The 2xxx alloys achieve the highest fatigue resistance and damage tolerance of any aluminium system, making them the primary choice for aerospace fuselage skin, wing lower surface panels, and damage-tolerant applications. The key alloys:

• 2024-T351: 4.35% Cu, 1.5% Mg, 0.6% Mn. The baseline aerospace structural alloy. R_p0.2 ≈ 325 MPa, UTS ≈ 470 MPa. Superior fatigue crack propagation resistance compared to 7075 due to crack closure effects from the S′ (Al₂CuMg) precipitate. Used for fuselage skin, lower wing skin, wing stringers. Susceptible to general and intergranular corrosion; requires cladding (Alclad 2024) or anodising for corrosion protection.
• 2014-T6: 4.4% Cu, 0.8% Si, 0.8% Mn. Higher strength than 2024-T6 (R_p0.2 ≈ 415 MPa) due to combined θ′ and Si strengthening. Used for aircraft spars, fuselage frames, and military aerospace. Also available as 2014A (BS standard).
• 2219-T87: 6.3% Cu, 0.3% Mn, 0.18% Zr, 0.1% V, 0.06% Ti. Designed for elevated-temperature service (up to 175°C sustained) and cryogenic applications (LH₂ tanks for aerospace launch vehicles). The T87 temper (solution HT + 7% cold work + artificial age) achieves ~455 MPa UTS. One of the few 2xxx alloys with acceptable weldability using 2319 filler.
• 2195 (Al-Li-Cu): Contains 4.0% Cu and 1.0% Li. The T8P4 temper achieves >560 MPa UTS with density 2.71 g/cm³ — the same as 2024 despite the lithium addition reducing density 3% per 1% Li. Used for the Space Shuttle external tank’s light-weight tank (LWT) and subsequent cryogenic structures.

3xxx Series — Aluminium-Manganese

Manganese additions (up to 1.5%) provide moderate strengthening through Mn-rich dispersoids (Al₆Mn, Al₁₂Mn₃Si) that pin subgrain boundaries and grain boundaries, refining the recrystallised grain structure and improving creep resistance at moderately elevated temperatures. Alloy 3003 is the most widely produced aluminium alloy by volume after 1100 — used for cooking utensils, chemical equipment, and packaging. Alloy 3004 (with 1.0% Mg) is the primary body sheet for beverage cans (drawn and ironed wall process); 3104 is the current commercial formulation. The 3xxx series is not heat-treatable and achieves maximum UTS of approximately 200 MPa in H18 temper.

4xxx Series — Aluminium-Silicon

Silicon (up to 12%) dramatically reduces the melting point of aluminium (the Al-Si eutectic is at 12.6% Si, 577°C), making 4xxx alloys ideal for brazing fillers (4047: 12% Si), welding filler wires (4043: 5% Si, the most widely used aluminium filler metal), and casting (the 4xxx wrought system overlaps with the A3xx.x cast system). The 4032 alloy (12.5% Si, 1% Mg, 0.9% Cu, 0.9% Ni) is precipitation-hardenable and used for high-silicon piston alloys where low coefficient of thermal expansion is needed. Most 4xxx alloys are classed as non-heat-treatable for structural purposes, though 4032 is a notable exception.

5xxx Series — Aluminium-Magnesium

The 5xxx series combines good strength (from Mg solid-solution hardening), excellent corrosion resistance (particularly in seawater), and the best weldability of any aluminium alloy family. Magnesium is the most effective solid-solution strengthener in aluminium per unit weight, providing approximately 40 MPa per 1% Mg addition. Key alloys:

• 5052-H32: 2.5% Mg, 0.25% Cr. R_p0.2 ≈ 195 MPa. The standard alloy for aircraft fuel tanks, marine applications below Mg 3.5% threshold, and sheet metal fabrication where good formability is needed.
• 5083-H116/H321: 4.5% Mg, 0.7% Mn, 0.15% Cr. R_p0.2 ≈ 215 MPa. The preferred alloy for marine hull construction, cryogenic LNG storage tanks (excellent toughness to −196°C), and pressure vessels. H116 and H321 are special tempers specifying controlled cold work and stabilisation to ensure the alloy is not in a sensitised condition.
• 5182-H19: 4.5% Mg, 0.35% Mn. Beverage can end stock (the pull-tab ring material). High work-hardening rate ensures the can end maintains structural rigidity despite very thin gauge (0.22 mm).
• 5454-H32: 2.7% Mg, 0.75% Mn. The highest-Mg 5xxx alloy approved for sustained elevated-temperature service (≥65°C); used for automotive tanker truck bodies carrying chemicals and fuels, where sensitisation of higher-Mg alloys would be a concern.

6xxx Series — Aluminium-Magnesium-Silicon

The 6xxx series is the most versatile and widely used family for structural extrusions, vehicle body parts, bridge and building sections, and general-purpose medium-strength applications. The Mg-Si ratio is critical: alloys balanced at the Mg₂Si stoichiometry (Mg:Si ≈ 1.73:1 by weight) achieve maximum T6 strength from β′′ precipitation. Excess Si beyond stoichiometry increases strength (Si participates in additional precipitates) but reduces fracture toughness. Excess Mg reduces strength slightly but improves corrosion resistance.

• 6061-T651: 1.0% Mg, 0.6% Si, 0.28% Cu, 0.2% Cr. R_p0.2 ≈ 276 MPa, UTS ≈ 310 MPa. The most widely specified medium-strength structural aluminium alloy globally. Available in all product forms; used for structural members, machine parts, truck frames, towers, and pipework. Loses ~40% yield strength in weld HAZ.
• 6063-T5/T6: 0.7% Mg, 0.4% Si. Lower strength (R_p0.2 ≈ 215 MPa T6) but excellent surface finish after anodising. The standard architectural extrusion alloy for window frames, curtain wall systems, and decorative sections where surface appearance is critical.
• 6082-T651: 1.0% Mg, 1.0% Si, 0.7% Mn. The European equivalent of 6061 (EN AW-6082) with higher Mn content providing better fatigue performance. R_p0.2 ≈ 260 MPa. Preferred for bridges and structural welded fabrications in Europe.
• 6005A-T61: 0.7% Mg, 0.6% Si (balanced composition). High extrudability for complex thin-wall sections; used for railway carriage floor extrusions and automotive crash management systems.

7xxx Series — Aluminium-Zinc

The 7xxx series contains the highest-strength commercial aluminium alloys, achieving yield strengths exceeding 600 MPa in some advanced compositions. Zinc (up to 8.2%) and magnesium (up to 3.0%) combine to form the η′ (MgZn₂) strengthening phase. Copper additions (in 7075, 7010, 7050, 7068) dramatically improve SCC resistance. Zirconium (0.08–0.15%) is added in modern alloys (7010, 7050, 7085) to form fine Al₃Zr dispersoids that control recrystallisation and maintain an unrecrystallised fibrous grain structure, which improves fracture toughness in thick sections — a critical requirement for aerospace primary structure.

• 7075-T651: 5.6% Zn, 2.5% Mg, 1.6% Cu, 0.23% Cr. The classic high-strength alloy. R_p0.2 ≈ 503 MPa, UTS ≈ 572 MPa. Used for aircraft wing spars, fuselage frames, machined structural fittings, and sports equipment (bicycle frames, climbing carabiners). SCC-susceptible in T651; T7351 temper for SCC-critical applications.
• 7050-T7451: 6.2% Zn, 2.3% Mg, 2.3% Cu, 0.12% Zr. Designed for thick plate (>75 mm) where quench rate sensitivity of 7075 causes strength loss in the core. Zr dispersoids suppress recrystallisation. R_p0.2 ≈ 455 MPa (T7451, thick plate). Standard alloy for large aircraft structural frames and thick-section forgings.
• 7085-T7452: High Zn (7.2%), moderate Cu (1.6%), Zr. Developed for very thick (100–200 mm) structural forgings with improved through-thickness fracture toughness and fatigue performance. Fuselage frames for wide-body aircraft.
• 7068-T6511: 7.2% Zn, 2.5% Mg, 2.2% Cu, Zr. Currently one of the highest-strength 7xxx extrusion alloys; R_p0.2 ≈ 620 MPa in T6511 extrusion. Used for rock climbing equipment, firearms components, and high-performance sporting goods requiring maximum strength in an extruded form.

Cast Aluminium Alloy Designations

The three-digit system for cast aluminium alloys, also administered by the Aluminum Association and adopted in ASTM B179 and ISO 3522, uses a different format from the wrought system. A letter prefix (A, B, C, D, or none) denotes compositional modifications to the original alloy, followed by three digits identifying the series and specific alloy, followed by a decimal point and one digit indicating the product form (x.0 = casting, x.1 or x.2 = ingot for remelting).

Principal Cast Alloy Series

The most commercially significant cast series is 3xx.x (Al-Si-Cu/Mg), which accounts for approximately 80–90% of all aluminium castings by volume. Silicon provides excellent castability (fluidity, feeding of shrinkage, reduced hot cracking tendency) by lowering the liquidus and increasing the freezing range. The three key cast alloy families within 3xx.x are:

• A380.0: Al-8.5Si-3.5Cu. The most widely die-cast alloy. Excellent castability (thin sections, complex geometry), good machinability. Used for automotive transmission housings, engine brackets, and electronic enclosures. Not heat-treatable; limited corrosion resistance in coastal environments without surface treatment.
• A356.0-T6: Al-7Si-0.3Mg. The premium gravity and low-pressure die cast alloy for structural applications. Heat-treatable: T6 treatment (β′′ Mg₂Si precipitation) achieves R_p0.2 ≈ 200–220 MPa. Used for automotive wheels, suspension knuckles, and aerospace castings. Modified with strontium or sodium to refine the eutectic silicon from coarse plates to fine fibres, dramatically improving elongation (from ~2% to ~8–12%).
• A319.0: Al-6Si-3.5Cu. Intermediate between 380 and 356; used for engine blocks and cylinder heads where a balance of castability, machinability, and moderate elevated-temperature strength is needed. Copper addition provides θ-CuAl₂ precipitate strengthening at engine operating temperatures.

The 2xx.x series (Al-Cu cast alloys) — particularly A201.0-T7 — achieves the highest strength of any aluminium casting (R_p0.2 ≈ 415 MPa), used for aerospace investment castings and structural housings. However, Al-Cu cast alloys have poor castability (narrow freezing range, high hot-cracking tendency) and require very controlled foundry practice. The 5xx.x series (Al-Mg) is used for marine and corrosion-resistant castings (A514.0, A535.0) where corrosion resistance in seawater is more important than strength.

Aluminium Alloy Welding Metallurgy

Aluminium welding presents metallurgical challenges fundamentally different from steel welding. The oxide Al₂O₃ film on aluminium melts at 2,050°C — far above the aluminium melting point of 660°C — and must be disrupted by the arc before fusion can occur. GMAW and GTAW use alternating-current (GTAW) or DC electrode-positive (GMAW) arc cleaning action to mechanically disrupt the oxide. The other major challenges are:

Porosity

Hydrogen is the principal cause of porosity in aluminium welds. Liquid aluminium has high hydrogen solubility (0.69 ml/100 g at 660°C), while solid aluminium has near-zero solubility (0.036 ml/100 g). As the weld pool solidifies, dissolved hydrogen is rejected and forms pores if it cannot escape before solidification is complete. Sources of hydrogen include: surface oxide and hydroxide on the base metal and filler (hence the importance of degreasing and wire brushing immediately before welding), moisture in the shielding gas system, atmospheric humidity, and hydrocarbon lubricants on wire. Best practice is to clean the joint immediately before welding with a stainless steel wire brush (not carbon steel) and to use dry, high-purity shielding gas (≥99.998% Ar).

HAZ Strength Loss in Heat-Treatable Alloys

The weld thermal cycle dissolves the strengthening precipitates in the HAZ over a width of approximately 10–25 mm, reducing the local yield strength to near the annealed (O temper) value. For 6061-T6, this means HAZ yield strength of approximately 110–130 MPa against base metal R_p0.2 of 276 MPa — a 50% reduction. Post-weld heat treatment (solution anneal + T6 ageing) can restore full properties, but this requires specialised furnaces and is impractical for large welded assemblies. AWS D1.2 (Structural Welding Code — Aluminium) accounts for this by requiring the designer to use reduced allowable stresses in the HAZ region. Friction stir welding (FSW) is strongly preferred for 2xxx and 7xxx alloys because the lower process temperature (below solidus) dissolves fewer precipitates and results in a narrower, stronger HAZ.

Solidification Cracking

Hot cracking (solidification cracking) is the primary risk in aluminium welding, particularly for 2xxx and 7xxx alloys with wide solidification temperature ranges. Solidification cracking occurs when tensile stress is applied to the mushy zone (liquid + solid) before complete solidification; the persistent liquid film at grain boundaries tears open. The crack susceptibility depends on alloy composition: the worst compositions are those that produce wide, highly viscous mushy zones. The remedy is filler metal selection: 4043 (Al-5Si) or 4047 (Al-12Si) fillers dilute the weld with silicon, shifting the solidification behaviour toward the low-cracking-risk eutectic point. AWS A5.10 provides filler metal selection guidance; the filler wire must be compatible with the base metal alloy to avoid formation of low-melting eutectics.

Engineering Selection Guide

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